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MAX4995AL Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX4995AL Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 13 page 50mA to 600mA Programmable Current-Limit Switches ______________________________________________________________________________________ 11 Applications Information Setting the Current Limit A resistor from SETI to ground programs the current-limit value for the MAX4995. Table 2 lists various current lim- its set by different resistor values at SETI. Shorting SETI to ground asserts FLAG. Use the following formula to calculate the current limit: Using an RSETI with a value smaller than 45.8k Ω results in a higher current limit. A programmed output current greater than 660mA can damage the device. Connecting any capacitance larger than 20pF to SETI can cause instability. Input Capacitor Connect a capacitor from IN to GND to limit the input voltage drop during momentary output short-circuit conditions. Use a 1µF minimum ceramic capacitor for proper device operation. Larger capacitor values reduce the voltage undershoot at the input. Due to the very fast current-limit reaction time of the MAX4995AF, a larger input capacitance might need to be connected at the input to dampen oscillation due to long wires. Choose a value large enough to ensure IN doesn’t exceed the absolute maximum ratings. Output Capacitor For stable operation over the full temperature range and over the full programmable current-limit range, use a 1µF ceramic capacitor from OUT to ground. If the load capacitance is too large, then current may not have enough time to charge the capacitance and the device assumes that there is a faulty load condition. Calculate the maximum capacitive load (CMAX) value that can be connected to OUT using the following formula: For example, for VIN = 3.3V, tBLANK(MIN) = 10ms, and ILIM = 300mA, CMAX equals 909µF. Due to the very fast current-limit reaction time of the MAX4995AF, a larger output capacitance might need to be connected at the output to dampen oscillation due to long wires. Choose a value large enough to ensure OUT doesn’t exceed the absolute maximum ratings. Layout and Thermal Dissipation To optimize the switch response time to output short- circuit conditions, it is very important to keep all traces as short as possible to reduce the effect of undesirable parasitic inductance. Place input and output capacitors as close as possible to the device. IN and OUT must be connected with wide, short traces to the power bus. During normal operation, the power dissipation is small and the package temperature change is minimal. If the output is continuously shorted to ground at the maxi- mum supply voltage, the operation of the switches with the autoretry option does not cause problems because the total power dissipated during the short is scaled by the duty cycle: Attention must be given to the MAX4995C continuous current-limit version when the power dissipation during a fault condition may cause the device to reach thermal shutdown threshold. P VI t tt MAX IN(MAX) OUT(MAX) BLANK RETRY BLANK = ×× + C( F) I (mA) t (ms ) V( MAX LIM BLANK(MIN) IN μ= × V) R(k ) 2 I( A 2.48 k SETI LIM ΩΩ = − 9042( ) ) () V m RSETI (k Ω) TYPICAL CURRENT LIMIT (mA) 45.8 602 55.6 500 70.6 397 94.2 300 143 200 191 150 287 100 576 50 ∞ (Open) 0 Table 2. Current Limit vs. Resistor Values |
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